The precentral gyrus is a strip of cortex on the outer surface of each hemisphere, at the back edge of the frontal lobe: it runs from the top of the head down the side of the brain, just in front of the central sulcus. [1]
The 3D brain needs JavaScript. The text below still says where the precentral gyrus is and what it does.
Slice
Opens cut by a top (axial) slice through the precentral gyrus. Drag to turn, pinch or scroll to zoom, tap any part to name it; .
A prominent gyrus on the surface of the back (posterior) part of the frontal lobe, mostly on the outer, convex side of each hemisphere. [1]
Behind it the central sulcus separates it from the postcentral gyrus; in front it is bounded by the precentral sulcus, and at its lower end by the lateral sulcus (Sylvian fissure). [1]
On the inner (medial) surface of the hemisphere it is continuous with the paracentral lobule. [1]
Strictly, the primary motor cortex lies on the front wall of the central sulcus and extends partly onto the precentral gyrus; the front of the gyrus holds premotor areas. [2]
What it does
It is the site of the primary motor cortex (Brodmann area 4), the primary region of the motor system. [1][2]
OpenStax: the precentral gyrus is the primary motor cortex; its cells are the upper motor neurons that instruct cells in the spinal cord to move skeletal muscles. [3]
The body is mapped along it, upside down (the motor homunculus): the leg close to the midline, then from top to bottom the trunk, shoulder, arm, hand and fingers, eyelids, lips and jaw. Hands, lips and face get far more cortex than their size suggests. [2]
Each hemisphere’s primary motor cortex holds a map of the opposite side of the body only. [2]
Connections
Its long fibres descend as the corticospinal tract to the spinal cord, and to the motor nuclei of the cranial nerves on the opposite side; most cross over in the brainstem. [2][4]
It works with other motor areas: the premotor cortex, the supplementary motor area, the posterior parietal cortex and several deep structures, to plan and execute voluntary movements. [2]
Inside it
Layer V of its cortex holds Betz cells, giant pyramidal neurons 70–100 μm across; they mark the primary motor cortex but supply only a small share of the corticospinal tract. [2]
Other names
Primary motor cortex, motor strip or M1; Brodmann area 4. [2]
Size
On this model the left precentral gyrus measures about 66 mm front to back, 66 mm top to bottom and 62 mm side to side (the box it fits in, not its volume); the right one is its mirror image. [5]
Centre of the left precentral gyrus compared with the centre of the left postcentral gyrus: 15 mm further forward; 15 mm apart in a straight line. [5]
Centre of the left precentral gyrus compared with the centre of the left prefrontal cortex: 49 mm further back, 16 mm higher, 8 mm further from the midline; 52 mm apart in a straight line. [5]
Facts describe anatomy and normal function only. The 3D model is the adult male BodyParts3D brain (credits and licence); real brains vary in size and shape, so its measurements describe one model, not anyone’s brain.